Backport relevant changes from gitlab-org/gitlab-ee!51

To do:

- [x] Update gitlab-shell

See merge request !1822

Fix github importer to handle empty issues

Documentation for Git LFS

Doc explaining how to use Git LFS, how to use Git LFS with GitLab, known issues and troubleshooting tips.

See merge request !1813

More labels since we get more specializations.

/cc @creamzy @ernstvn @dzaporozhets 

See merge request !1821

Correctly set comparison first commit when range includes a merge commit

Fixes #3207.

See merge request !1814

[ci skip]

See merge request !1705

Add notification to the former assignee upon unassignment

Replaces !1769

Fixes #3395

See merge request !1818

Prevent the last owner of a group from being able to delete themselves
by 'adding' themselves as a master

Replaces !1708.

Fixes #1111.

See merge request !1815

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

[ci skip]

See merge request !1816

[ci skip]

[ci skip]

Don't render note edit forms for visitors